1. Field of the Invention
The present invention relates to non-porous films. More particularly, the present invention relates to the use of non-porous films in analytical elements whereby analytes in samples of whole blood are separated from substances that can interfere with the desired analysis.
2. Description of Related Art
Various analytical elements for clinical analysis have been proposed using "dry chemistry", i.e., analytical clinical techniques wherein chemical reagents are incorporated in various substantially "dry-to-the-touch" elements, such as test strips and multizone analytical test elements.
U.S. Pat. No. 3,992,158 issued Nov. 16, 1976 discloses improved multizone analytical elements for the analysis of liquids via "dry chemistry". In the element disclosed, there is present a reagent zone and a spreading zone. When the spreading zone of the element was spotted with a sample of aqueous liquid to be tested for a particular analyte, this zone was found to be especially effective in receiving the liquid test sample and distributing such liquid within itself in a manner to provide a substantially uniform concentration of analyte for interaction with the reagent components of the element. The reagent zone contains any of a variety of compositions interactive with the desired analyte to provide, for example, a radiometrically detectable species indicative of the presence and/or concentration of the analyte. These analytical elements are used in the analysis of a variety of blood components including glucose, uric acid, and protein, e.g. albumin.
In these elements, the spreading zone pore size is generally chosen so that the zone filters out undesirable sample components that interfere with an analytical reaction or with detection of any test result produced within the element. It is taught that for analysis of whole blood, porous zones having a pore size of from 1 to about 5 microns are particularly useful in screening out blood cells. Also, a filter or dialysis zone can be included at an appropriate location in the element. It is stated that in analyzing for blood glucose, a dialysis zone, such as a semi-permeable cellulose membrane, can prevent passage of proteins and other potentially interfering substances to the reagent zone.
U.S. Pat. No. 4,258,001 issued Mar. 24, 1981, describes elements that have, as a spreading layer, a particulate structure comprising (1) a plurality of heat-stable, organopolymeric particles non-swellable in, and impermeable to, the liquid under analysis and (2) an adhesive for these particles. These layers can be used along with one or more separate functional zones permeable to the liquid under analysis, such as reagent zones, registration zones, radiation-blocking zones, and selectively permeable barrier zones. These optional zones can include, among others, filtering zones to filter out or remove particular components of applied liquid samples, as described in U.S. Pat. No. 3,992,158, and barrier compositions having a predetermined selective permeability permitting only selected species to come into fluid contact with particular zones of the multizoned element, such barrier compositions being described in U.S. Pat. No. 4,066,403 issued Jan. 3, 1978. These elements can be used to analyze either serum, plasma, or whole blood. The barrier compositions described in U.S. Pat. No. 4,066,403 that are specifically mentioned therein are cellulose acetate butyrate, cellulose propionate valerate, poly(methylmethacrylate), and cellulose acetate.
Published European Patent Applicaton No. 10425 is directed to a semi-permeable composite membrane comprising a thin semi-permeable film of a polymeric material deposited on one side of a microporous substrate. The polymeric material is prepared by cross-linking a soluble polymer containing at least 30 mole% of a recurring unit of the formula ##STR5## wherein, for example, among a number of possibilities,
Y can be a carbonyl group ##STR6##
R.sub.1 can be a hydrogen atom or a monovalent organic radical containing 1 to 20 carbon atoms;
R.sub.2 can be a hydrogen atom;
R.sub.3 can be an alkylene group containing 2 to 5 carbon atoms; and
R.sub.4 can be a hydrogen atom or a methyl group. Cross-linking is carried out through the amino group by means of a polyfunctional compound.
These membranes can be used for reverse osmosis. It is said that reverse osmosis can be used for removing impurities from liquids such as water or blood. The polymer used in preparing the semi-permeable membranes can be a copolymer containing up to 70 mole% of one or more comonomers. These polymers can be blended with up to 50 mole % of another polymer having the ability to form a water-soluble or aqueous emulsion. The polymer is deposited on a microporous substrate by applying a solution of the base polymer to the substrate. After such deposition, the polymer is cross-linked.
Recently, the Fuji Photo Film, Co., Ltd., has described multizone elements for blood analysis in the patent and technical literature, e.g., Clinical Chemistry, Vol. 27, No. 7, 1981, 1287-1290; U.S. Pat. No. 4,256,693; U.S. Pat. No. 4,255,384; and U.K. patent application Nos. 2,069,131 A and 2,069,132 A, both published Aug. 19, 1981.
The Clinical Chemistry article describes an element for determining glucose in whole blood. The element consists of a spreading zone, a blocking zone, an enzymic reagent zone, and a transparent support zone. It is said that blood cells and platelets are filtered off through the fibrous structure of the spreading zone, but that blood plasma diffuses through to the blocking zone. Diffusion of macromolecules and hydrophobic substances bound to proteins in a plasma are reported to be blocked at the blocking zone, thereby not reaching the reagent zone. It is also said that the blocking zone further serves as a reflector for reflection densitometry and a shield from interference by colored materials in blood such as hemoglobin and bilirubin on the spreading zone.
Japanese Kokai 129790/79 relates to a filter for separating leucocytes that comprises a polymer containing at least 1.0% by weight of (A) and/or (B) below. ##STR7## wherein
R.sub.1 represents H or CH.sub.3 ; R.sub.2,
R.sub.3, and R.sub.4 each represents H, CH.sub.3,
C.sub.2 H.sub.5 or C.sub.3 H.sub.7 ; X represents Cl, OH,
Br, I or NO.sub.3 ; and n is an integer of 1 to 30, the filter being packed with the polymer.
As a comonomer there can be used any monomer than can be copolymerized with at least 1% by weight of (A) or (B). In use, the polymer is fashioned into a thread-like or granular form and placed in a column. The leucocytes remain in the filter and the erythrocytes are hemolyzed and pass through the filter along with the plasma.
Other semi-permeable membranes capable of separating desired substances from the many components of whole blood have also been described, including gelatin (U.K. Published Application No. 2,069,131 A), cellulose or cellulose derivatives (U.K. Pat. No. 911,181), and ethyl cellulose (U.K. Pat. No. 922,665).
Many of these materials, as well as others (see Examples 5-8, infra), were tested by us in an effort to provide an improved element of the "dry assay" type for the determination of analytes, particularly glucose, in whole blood. None, however, were satisfactory. In some instances, the membranes were too permeable; in others, they were not permeable enough. In either case, the result was an inability to differentiate meaningfully among various analyte concentration levels. Thus, a need remains for analytical elements of the type used in so-called "dry assays", that are improved by the inclusion therein of barrier zones particularly suited for separating analytes from the several interferents present in whole blood.